In known coextrusion heads, the layers of synthetic resin are joined in stages, are transported together as their volume increases, and then are combined into one single tube, which is extruded from the head's nozzle. This method of production can result in a nonuniform tube structure, in which the adjacent layers buckle, have an uneven thickness, are separated, or the converse, become intermixed.
That a wide range of uses and articles can be produced, by blow molding, from multilayered parisons is due to the large number of arrangements and compositions of the individual layers. The introduction of a barrier layer expands the range of uses to those where it is necessary to prevent the diffusion of the contents, or portions of the contents, through the finished container. The arrangement and composition of the barrier layer depends upon the material whose escape is to be prevented. In addition, article strength, resistance and cost play important roles, as does esthetics, in the composition of the finished container. For instance, the desired outside luminosity, or glow, might influence the type and positioning of one or more layers in the composite. Generally the barrier layer, consisting of materials such as ethylene-vinyl-alcohol (EVOH) or polyamide (PA), and the supporting layers, for example made of polyethylene (PE) or polypropylene (PP), may have different physical properties which include differing flow characteristics. The adhesive layer, on the other hand, generally has the same basic molecular structure as the associated supporting layer. As a result, these two layers generally have similar physical properties to include similar flow characteristics.
In known coextrusion heads, the annular passageways, one for each synthetic resin layer, are spaced apart from one another where they join the inner main feed passageway. As an example, a "standard" coextrusion head will first have the inner supporting layer fed into the main passageway. Next, an inner adhesive layer is added and then the barrier layer material is joined to the flow. Before the outer adhesive layer, and subsequently, the outer supporting layer, is fed into the flow, the laminate consisting of the inner supporting layer, the inner adhesive layer and the barrier layer are transported a considerable distance in the feed passageway. During this passage, the inner supporting layer and the barrier layer are in contact with the boundary walls of the feed passageway. The different flow characteristics of these two layers lead to high shear stresses both within each layer and between the individual layers. The aforementioned structural defects within the extruded tube are the result of these shear stresses. Thus, the articles produced from such extruded tubes are of a lower quality.
U.S. Pat. 4,522,775, issued June 1, 1985, discloses a coextrusion head of this known type. The various synthetic resin layers are fed into the inner main feed passageway at an axial spacing from one another. In addition, both adhesive layers and the barrier layer are joined prior to the merger with the inner supporting layer. Although the disclosed configuration may simplify the joining of the thin barrier layer and its inner and outer adhesive layers with the quite thick inner supporting layer, it does not avoid the possible structural defects resulting from the different flow characteristics of the adhesive layers and the barrier layer. The disclosed coextrusion head continues to transport a laminate formed of an adhesive layer and a barrier layer a considerable distance within a feed passageway thereby introducing the shear stresses resulting from differing flow characteristics as the inner adhesive layer and the barrier layer each engage one of the boundary walls of the feed passageway as they move toward the junction where the outer adhesive barrier is joined.